A method is known by which the surface of a work member such as a transparent glass substrate is marked by utilizing ablation by a laser beam. With this method, fine cracks are formed on the surface of a work member and broken pieces are sometimes mixed with another work member on a manufacture line. Since deposits called debris are formed near the marked area, it becomes necessary to clean and remove such deposits.
A method is disclosed in JP-A-HEI-3-124486 by which a marker is formed in a work member by converging a laser beam at the inside thereof, without damaging the surface of the work member. With this method, since the surface of a work member is not damaged, it is possible to prevent formation of fine cracks and attachment of debris.
According to the method disclosed in JP-HEI-A-3-124486, when the energy density of a laser beam exceeds a breakdown threshold value of a work member, at the converging point of the laser beam or in its nearby area, fine parts of the work member are destructed and a marker is formed. By disposing fine markers in two- or three-dimensionally, characters or figures can be drawn. By narrowing a pitch between disposed fine markers, the contrast of characters or figures can be improved.
With this method, however, it is difficult to control the shape of a fine marker formed through destruction of a work member. The shapes of markers become, therefore, irregular. If adjacent markers contact each other, the mechanical strength of the work member lowers. It is therefore preferable to dispose markers in a dispersed manner. In order not to make adjacent markers contact each other even if the shapes of markers are irregular, it is necessary to increase the pitch between adjacent markers and provide a margin to some extent.